The present invention relates to a belt transfer device applied to an apparatus for electrophotographic image formation, which includes an image carrier. The belt transfer device includes an intermediate transfer belt made of elastic material. The belt transfer device primarily transfers a toner image on the image carrier to the intermediate transfer belt and secondarily transfers the image on the belt to a sheet of paper or another record medium such as an OHP sheet.
Some apparatuses for electrophotographic image formation such as printers and copiers include an image carrier and a belt transfer device, which includes an intermediate transfer belt made of elastic material and a transfer member. The intermediate transfer belt runs in a loop. The transfer member can shift toward the image carrier so as to bring the intermediate transfer belt into compressive contact with the carrier. The belt transfer device primarily transfers a toner image on the image carrier to the intermediate transfer belt and secondarily transfers the image on the belt to a sheet of paper. The length of the intermediate transfer belt depends on the size of the largest sheets on which the associated apparatus can form images.
Because the sheet size series for frequent use with apparatuses for image formation vary with their various destinations, intermediate transfer belts of different lengths are provided for the destinations. Because the intermediate transfer belt of an apparatus for image formation deteriorates with time as the apparatus repeats image formation, the belt needs to be replaced when the apparatus has repeated image formation a specified number of times.
The tensile force of the intermediate transfer belt fitted to the belt transfer device of an apparatus for image formation varies with the length of the belt. The tensile force variation varies the width of the nip formed between the intermediate transfer belt and the image carrier of the apparatus when the transfer member of the belt transfer device brings the belt into compressive contact with the carrier. The nip width variation varies the toner image transfer performance of the apparatus. Therefore, the monochromatic unit of the belt transfer device includes a tension member for applying, to the intermediate transfer belt, tensile force according to the length of the belt.
A belt transfer device is also used in a tandem apparatus for color image formation, which includes image carriers arrayed in a line. The image carriers are a monochromatic image carrier for carrying a monochromatic toner image and three color image carriers for carrying toner images of the three primary colors. The tandem apparatus forms monochromatic images more frequently than color images. The monochromatic image carrier is larger in diameter than the color image carriers so that the lives of all the carriers can be equal, and so that the tandem apparatus can form monochromatic images at a higher speed than color images. Each type of tandem apparatus for color image formation is fitted with a monochromatic image carrier of a diameter according to the speed at which the apparatus is required to form monochromatic images. By contrast, because it is strongly demanded that tandem apparatuses for color image formation form color images of high quality, the color image carriers of the apparatuses have a diameter common to the apparatuses.
In a tandem apparatus for color image formation, the distance between the monochromatic image carrier and each of the color image carriers depends on the diameter of the monochromatic image carrier. JP-A-2004-109267 discloses a conventional belt transfer device including a color unit and a monochromatic unit. The color unit supports color transfer members. The monochromatic unit supports a monochromatic transfer member, which primarily transfers a monochromatic toner image. Monochromatic units of some types are provided for different diameters of monochromatic image carriers. The belt transfer device of a tandem apparatus for color image formation includes a combination of a color unit of the single type and a monochromatic unit suitable for the diameter of the monochromatic image carrier of the apparatus. The two units are fitted to the main frame of the belt transfer device.
The process for producing an intermediate transfer belt includes injection-molding an elastic material into an endless belt, extending the endless belt to a specified length in a heating mold, and cooling the extended belt. While the belt is heated and cooled during the production process, its thermal deformation is liable to produce an error in the length of the belt. If a wrong intermediate transfer belt is fitted to the belt transfer device of a tandem apparatus for color image formation during the assembly or maintenance of the apparatus, the tensile force of the belt is not proper. This lowers the toner image transfer performance of the apparatus, resulting in a deterioration in image quality.
The object of the present invention is to provide a belt transfer device for an apparatus for image formation, the transfer device making it possible to accurately determine whether the tensile force of its intermediate transfer belt is proper, keeping the belt under constant tension so as to maintain good image quality, and avoiding being fitted with an intermediate transfer belt of a length different from that suitable for the destination of the apparatus.